1. Field of the Invention
The invention relates to analog to digital converters generally and to the so-called delta-sigma modulators more specifically.
2. Description of the Prior Art
Modern information transmission systems are often based on the conversion of analog input signals to digital signals for transmission over a discrete channel. Both the analog to digital and digital to analog subsequent reconstruction are subject to errors because the continuum of possible input values must be represented by discrete sets of values in the transmission channel. This error is commonly known as quantizing noise and constitutes one of the major sources of inaccuracies in such systems.
In a technical paper (herein called paper No. 1) entitled "Reduction of Quantizing Noise by Use of Feedback," H. A. Spang III and P. M. Schultheiss analysed the quantizing noise problem and suggested the use of quantizer feedback as a means of ameliorating the inaccuracies produced by this quantizing noise. That paper appeared in the IRE Transactions on Communication Systems, Vol. CS-10, pp. 373-380 (December 1962). Therein, a general case delta-sigma modulator having a multi-level quantizing characteristic is discussed and analyzed. Presumably, for simplicity of analysis, the authors of that technical paper have chosen to show the sampler and quantizer portions of the delta-sigma modulator as distinct entities. A physical configuration of that type cannot be implemented by conventional analog circuitry. An example of a conventional analog delta-sigma modulator is shown and described in a paper (hereinafter called paper No. 2) entitled "A Telemetering System by Code Modulation - Delta-Sigma (.DELTA..SIGMA.)Modulation - Delta-Sigma (.DELTA..SIGMA.) Modulation" by H. Inose, Y. Yasuda and J. Murakami. That paper was published in the IRE Transactions Space Electronics Telemetry, Vol. SET-8, pp. 204-209 (September 1962). In this latter paper the prior art, basically analog, delta-sigma (.DELTA..SIGMA.) modulator is presented and its utility as an improvement over the prior art delta modulator is described.
The authors of the latter paper have pointed out that in the still earlier, so-called delta modulation system, pulses are sent over a transmission line carrying information corresponding to the derivative of the input signal amplitude. At the receiving end, those pulses are integrated to obtain the original waveform. Transmission disturbances such as noise, etc., result in a cumulative error as a transmitted signal is integrated at the receiving end.
The so-called delta-sigma modulation system provides for integration of the input signal before it enters the modulator itself so that the output transmitted pulses carry information corresponding to the amplitude of the input signal.
In a basically analog implementation, the behavior of a delta-sigma modulator depends on the absolute value of capacitors and resistors in the circuits and, therefore, sensitivity to the detrimental effects of aging and temperature is encountered. Further, operational amplifiers included in analog implementations must be of superior quality, the gain and bandwidth of such amplifiers should not be such as to affect the transfer function of the integrator within the input circuitry of the device. Still further, in an analog implementation the waveform at the output of the digital to analog converter has to be precise and not pattern sensitive, that is, a pulse for an isolated "1" must be substantially identical to pulses imbedded in a series of "1-s". The circuit intricacies of analog delta-sigma modulator implementations are often the result of that requirement.
The manner in which the invention deals with these prior art disadvantages will be evident as this description proceeds. A detailed description of a digital delta-sigma modulator can be found in Applicant's U.S. Pat. No. 4,270,027, entitled "Telephone Subscriber Line Unit with Sigma-Delta Digital-to-Analog Converter", which patent is assigned to the same assignee as is the instant invention.